P. Bodart

Aluminium distribution and cation location in various M-ZSM-5-type zeolites (M Li, Na, K, Rb, Cs, NH4)

Combining PIGE, EDX, AA, and thermal analyses with solid-state 29Si-n.m.r spectroscopy allowed us to determine the amounts of SiOR and (SiOAI)−R+ groups (R H, M, TPA). The amount of SiOR increases with decreasing aluminium content. At the external surface of the crystallites, an Al-rich phase is deposited, which is revealed to be a zeolitic phase rather than a MAIO2 species. The d.t.a. peak at ca. 360°C is attributed to external shell TPA species. Finally, the partial solvation of the cations is shown by both 7Li- and 23Na-n.m.r. results.

Structure and stability of tetrapropyl-ammonium (TPA) ions in various alkali—aluminosilicate hydrogel precursors to crystalline (M)ZSM—5 zeolites

Abstract The stability of TPA species interacting with various (Al,Si) hydrogel environments in the presence of alkali cations has been evaluated by thermal analysis (combined t.g.-d.t.a.-d.t.g.). Three different TPA—gel associations have been characterized by d.t.a. endotherms : crystalline (TPAX) n clusters (X = OH − ,Br − ) which are also detected by XRD, monomeric (TPAX) species in direct association with the (Al,Si) gel, and labile entities, [TPA + …(H 2 O) x ] bulky water clathrates, that interact very weakly with the remaining negative charges available after neutralization of the gel by small (Li,Na,K) alkali cations. The formation, relative concentration and stability of these species essentially depend on the relative amounts of Al 2 O 3 and M 2 O present in the gel, as well as on the nature (structure forming/breaking) of the alkali cations. Monomeric as well as water clathrated species lead to crystalline (M)ZSM—5 upon heating.

Competitive Roles of Alkali and TPA Cations During Nucleation and Growth of ZSM-5 Zeolite

ZSM-5 was crystallized at 165°C in Teflon autoclaves from synthesis mixtures having the molar composition 60 SiO 2 - 1.2 Al 2 O 3 - 700 H 2 O and containing variable amounts of TPABr, TPAOH and MOH (M = Li, Na, K, Rb, Cs). The nature and the composition of the intermediate solid and liquid phases, as determined by combining a variety of (thermo) analytical and physicochemical techniques, is discussed in terms of competitive interactions between TPA + and M(H 2 O) n + cationic species occurring along the nucleation and growth processes. The size, chemical composition and TPA content of the final 100% crystalline ZSM-5 material formed in each system is strongly affected by intrinsic properties of the alkali cations.

GEL Composition Versus Organic Directing Agent Effects in the Synthesis of ZSM-39, ZSM-48 and ZSM-50 Zeolites

Abstract Zeolite ZSM-48 prepared from a hydrogel containing an alkylamine admixed with TMA + ions incorporates these organic species, so that the total filling of the porous volume is achieved. However, when zeolite ZSM-39 crystallizes from a similar hydrogel (also containing TMA + ions, and with or without alkylamine and aluminium ions), part of the TMA + ions are dequaternated to trimethylamine and trimethylammonium species, which are also occluded into the large cavities of the ZSM-39 crystals. Hydrogels including hexamethonium (HM ++ ) ions, with or without ammonium species, lead to the formation of zeolites ZSM-48 and ZSM-50, depending on the initial aluminium content. In all cases, the amount of organic species occluded (either only HM ++ ions, or HM ++ together with hexyltrimethylammonium and trimethylammonium ions which result from the decomposition of the HM ++ ions) is governed by the filling of the zeolitic free pore volume.

Role of Alkali and Tetrapropylammonium Cations in (M)ZSM-5 Hydrogel Precursors

The influence of alkali and tetrapropylammonium (TPA + ) cations on the organization of hydrogel precursors to ZSM-5 zeolite is examined by multinuclear high power solid state nuclear magnetic resonance techniques. Emphasis is put on the different states of the TPA + ions and on the interaction between the alkali cations (M + ) and the negative charges in the gel.

Nature and Structure of High Silica Zeolites Synthesized in Presence of (Poly) Alkyl Mono- and Diamines

Various aluminosilicate hydrogels having the composition 106 SiO 2 . 26 Na 2 O, Al 2 O 3 . 17.3 H 2 SO 4. . 47 NaCl. 2400 H 2 O. 9 R, where R is a mono- di- or trialkylamine, a diaminoalkane or a tetraalkylammonium (Alk 4 N + ) cation, have been left to crystallize in sealed Pyrex autoclaves at 140°C. The resulting crystalline zeolitic phaases were analyzed by a variety of techniques: PIGE, XRD, IR, SEM and solid state 29 Si- and 27 Al-NMR. Particular emphasis was put on the advantage to use in combination high resolution solid state 13 C-NMR and simultaneous TG-DTA-DTG techniques to characterize the nature and structure of the organic molecule entrapped within each zeolitic lattice. Typical examples showing the influence of the geometry and/or of the length of the alkyl chain(s) of the amine or Alk 4 N + cations, in directing either specific ZSM-5 or ZSM-11 structures or random ZSM-5/11 mixtures (or intergrowths), are presented and discussed.

Study of Mordenite Crystallization III: Factors Governing Mordenite Synthesis

Abstract Mordenite is synthesized from different aluminosilicate gels in sealed pyrex tubes at 165°C. Its crystallization domain is established for these conditions and the coexisting phases are identified. Differences with published data are explained assuming partial dissolution of the silica-rich pyrex glass. Morphologies and composition of the mordenite crystallites are related to the molar ratios of the reactants. In addition, the composition range of mordenite and the structure of coexisting phases (analcite, gismondine, silica polymorphs…) give information about the nature of some probable mordenite precursor species: single four-membered rings and single six-membered rings.

Reactivity Of Isopropanol On K- And Cs-Exchanged Zsm-5 And Mordenite

The effect of acidity and channel sizes of K- and Cs-exchanged ZSM-5 and mordenite on the reactivity of isopropanol is thoroughly investigated by 13 C-NMR in the adsorbed phase. The catalysts are characterized by X-ray diffraction technique, and by magic-angle-spinning high resolution solid state 29 Si-, 27 AI- and 13 C-NMR, as well as by proton induced γ-ray emission (PIGE) and EDX methods. TG analysis of adsorbed n-pentane leads to the determination of pore volume. For the ZSM-5 zeolites characterized by narrower channels with respect to the corresponding mordenites, the size of the cation has a definite influence on the reaction products. At 200°C, the acid catalyzed dehydration of isopropanol is essentially observed, leading to propylene. At 300°C, the higher basicitiy of Cs-ZSM-5 favours the formation of acetone, the dehydrogenation product of isopropanol.

Synthesis and Characterization of Faujasite-Type Zeolites II. The Role of Alkali Chlorides

Aqueous aluminosilicate hydrogels having the composition 20 Na 2 O. Al 2 O 3 . 20 SiO 2 . 700 H 2 O, heated at 100°C in polypropylene autoclaves, usually yield pure zeolite Y. The influence of various alkali and ammonium cations, added in varying amounts as chlorides to this initial hydrogel, was systematically examined. Increasing amounts of NaCl either favour the formation of larger crystals of zeolite Y or initiate a co-crystallization of zeolites X, Y and hydroxysodalite. In presence of either alkali chlorides, the crystallization of zeolite Y is totally inhibited while more compact zeolitic structures, such as zeolites M, P, ZK-19, W or Cs-pollucite, sometimes intermixed with parasite alkali aluminosilicates, are observed. The formation and the composition of these phases are connected with the structure-forming/breaking role of each specific alkali cation.